System for phosphoric acid production with decreased foaming

ABSTRACT

A system for the preparation of phosphoric acid from phosphate rock and sulfuric acid, includes in combination a first reaction vessel containing a first slurry comprising calcium sulfate hemihydrate, monocalcium phosphate and phosphoric acid, a second reaction vessel containing a second slurry comprising calcium sulfate hemihydrate, circulation means including a draft tube disposed centrally in each of said vessels and an agitator positioned axially in each of said vessels within said draft tube, a first conduit interconnecting said first and second reaction vessels for conducting said first slurry from said first reaction vessel to said second reaction vessel, a second conduit interconnecting said vessels for conducting said second slurry from said second reaction vessel to said first reaction vessel, an inlet pipe for introducing phosphate rock and phosphoric acid to said first reaction vessel, said inlet pipe connected with the interior of said draft tube in said first vessel and wherein a vent is connected to said inlet pipe to permit escape of gases and, thus, increasing the rate of flow of said phosphate rock and phosphoric acid to said first reaction vessel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to applications Ser. No. 703,139 issued Feb.20, 1979, as U.S. Pat. No. 4,140,748; Ser. No. 703,138 now abandoned;and Ser. No. 703,208, issued Jan. 2, 1979, as U.S. Pat. No. 4,132,760,all filed July 7, 1976; Ser. No. 676,559, filed Apr. 13, 1976, nowabandoned; Ser. No. 810,484, filed June 27, 1977, issued Jan. 23, 1979,as U.S. Pat. No. 4,136,199; Ser. No. 840,791, filed Oct. 11, 1977; tothe two applications of Ore's, Moore and Ellis filed Dec. 29, 1977, Ser.No. 865,557, issued Apr. 1, 1980, as U.S. Pat. No. 4,196,172 and Ser.No. 865,556, filed Dec. 29, 1977, now abandoned; and to our applicationSer. No. 866,990, now abandoned, titled "Apparatus for Phosphoric AcidProduction with Decreased Foaming"; and the two applications of Graggand Adams, Ser. No. 866,988, and Ser. No. 866,987, now abandoned, allthree filed on the same date as the subject application (all of theabove-cited applications are owned by Occidental Petroleum Corporationor its wholly-owned subsidiaries and the entire disclosures of which areincorporated herein by this reference).

BACKGROUND

The present invention especially relates to the processes described inapplication Ser. Nos. 676,559; 703,138; 703,139; and 703,208, since itpermits reaction or dissolution of phosphate rock with decreased foamingin all of these processes.

In these applications, phosphate rock and sulfuric acid are reactedunder conditions which result in the formation of solid calcium sulfate(either hemihydrate or gypsum) and phosphoric acid. A two vesselreaction system is used in which the reaction slurry undergoes intra-and inter- vessel circulation. Preferably, the intra-vessel circulationis through a draft tube. This results in excellent dispersion ofreactants and minimization of temperature and concentration gradientsthroughout the slurry. In the hemihydrate process, the solution portionof the slurry in the first vessel (the "dissolver") is preferablymaintained at a negative sulfate concentration (i.e. excess dissolvedCa⁺²) and the solution in the second vessel (the "crystallizer") ispreferably maintained at a positive sulfate ion concentration. Alsopreferred is that the second vessel be maintained at a reduced pressure(e.g. to provide evaporative cooling). Better filtration rates can thusbe obtained due to the favorable shape, dominant size and sizedistribution (especially, low fines content) of the calcium sulfatecrystals. Most preferred is that a crystal modifier (e.g. a sulfonicacid, a sulfonic acid salt, tall oil, fatty acids or esterified talloil, fatty acids) be present in the crystallizer. A two vesselhemihydrate process is described in U.S. Pat. No. 3,418,077 of Robinson,issued Dec. 24, 1968. Also relevant is the article by J. G. Getsinger,"IV Hemihydrate by the Foam Process" in Phosphoric Acid, Part 1, editedby A. V. Slack, at pages 369-382, Marcel Dekker, Inc., New York (1968),which shows a one vessel process.

U.S. Pat. No. 3,522,003 to Lopker shows CO₂ removal by venting but froma separate pipe from the feed inlet (see also U.S. Pat. No. 3,522,004).

The present invention is directed to the manufacture of phosphoric acidby the wet process wherein phosphate rock is dissolved or reacted inphosphoric acid to produce a solution or slurry of monocalciumphosphate. The hemihydrate, or as it is sometimes called, thesemihydrate, process is employed to produce wet phosphoric acid fromphosphate rock and sulfuric acid. Phosphate rock and phosphoric acid(which can contain H₂ SO₄ and is preferably pre-mixed in a separateslurry tank) are added to a first reaction vessel or set of reactors, inparallel or series, (the "dissolver") which contains a first slurrycomprising calcium sulfate hemihydrate, monocalcium phosphate, andphosphoric acid. The "soluble" or "excess" sulfate content (i.e., theexcess or deficiency of sulfate ions over calcium ions) of the firstslurry in the first reaction vessel can be maintained (e.g. by additionof sulfuric acid) at a concentration of about +0.7% to about -4% or even-8% (more preferred 0.0 to -6%), as determined for example, by thewell-known gravimetric analysis. It is more preferred to be at anegative sulfate (e.g. excess Ca⁺²). The sulfuric acid in the first(dissolver) vessel is usually contained in "recycle" phosphoric acidfrom a filtration step and/or sulfuric acid contained in a side streamor recycle slurry from the second reaction vessel. If two dissolvervessels are in series, it is preferred that slurry from the secondvessel (which has an excess of Ca⁺², therefore, no free sulfuric acid)is used as the dissolution medium in the first dissolver vessel (e.g.,the first vessel would be at about -6% SO₄ and the second vessel atabout -4%, caused by addition of sulfuric acid only to the secondvessel). This would greatly reduce the "lattice bound" P₂ O₅ loss. Thismethod of operation is the invention of James Moore, John Ellis and GaryBeer and will be the subject of a later filed application .

Sulfuric acid is added to the second reaction vessel which contains asecond slurry comprising calcium sulfate hemihydrate, monocalciumphosphate, sulfuric acid and phosphoric acid. The sulfuric acid reactswith the monocalcium phosphate and any residual, undissolved phosphaterock, producing calcium sulfate hemihydrate and phosphoric acid. Thesoluble or excess sulfate concentration of the second slurry ismaintained at a positive value (about +0.7% to about +4.5%).

Sulfuric acid is added in amounts such that the sulfate content of theadded acid and the sulfate content of the added rock is equivalent toabout 90% to about 100% (more preferred 93-99.5%) of the stoichiometricamount of sulfate required to react with calcium added in the phosphaterock to form calcium sulfate hemihydrate.

As is well-known in the art, sulfate and/or sulfuric acid can beintroduced as such or as a part of phosphoric acid "recycle" (as fromthe filtrate from filtering to separate the hemihydrate).

In order to maintain the desired soluble sulfate concentration in thefirst reaction vessel and in the second reaction vessel, circulationbetween the two reaction vessels is initiated. A first portion of thefirst slurry from the first reaction vessel is circulated through afirst conduit into the second reaction vessel, and a first portion ofthe second slurry from the second reaction vessel is circulated througha second conduit into the first reaction vessel. This circulation iscontinuous. In order to better disperse the added phosphate rock and theadded sulfuric acid within the slurry of the first and the secondreaction vessels respectively and to better disperse the incoming slurrywith the slurry present in the given reaction vessel, a second portionof the first slurry and a second portion of the second slurry iscirculated within the first and second reaction vessels, respectively,preferably each through its own draft tube preferably at a rate equal toat least 50% of the volume of the slurry in a given reaction vessel perminute. This inter- and intra- vessel circulation disperses thereactants within the slurry in the respective reaction vessels. A thirdportion of the second slurry is removed from the reaction system so asto separate the liquid and solid components from the said slurry.

Although such patents as U.S. Pat. No. 3,939,248 to Caldwell and U.S.Pat. No. 2,968,544 to Zietz show the uses of draft tubes in phosphoricacid processes, these patents are not concerned with the problems offeed tube gas blockage or foaming or with hemihydrate processes, whichare well-known to involve different deposition or scaling behavior thando gypsum processes.

Although the present process has been described as involving tworeaction vessels, it should be understood that additional vessels(including reaction vessels) can be useful in the process and, forexample, the dissolver can comprise two or more vessels in series or inparallel.

Especially preferred is the use of an additional vessel as a slurrytank, into which phosphoric acid (which can be a filtrate, usuallycontaining in the range of 0.5-3.5% sulfuric acid) and phosphate rockare contacted to form a slurry which is transported (as by an overflowpipe) to the dissolver vessel. In such a slurry tank, as in thedissolver, it is frequently useful to use venting and/or to add adefoaming agent. A preferred defoaming agent comprises tall oil, talloil fatty acids, or lower alkyl esters of tall oil fatty acids, ormixtures of such tall oil acids and esters, because such tall oiladditives can also function (alone or with added sulfonics) as crystalmodifiers. The preferred esters are the methyl and ethyl (as exemplifiedby the esters found in the commercial product marketed under thetradename AZ-10-A).

SUMMARY

The invention comprises a system for the preparation of phosphoric acidfrom phosphate rock and sulfuric acid, includes in combination a firstreaction vessel containing a first slurry comprising calcium sulfatehemihydrate, monocalcium phosphate and phosphoric acid, a secondreaction vessel containing a second slurry comprising phosphoric acidand calcium sulfate hemihydrate, circulation means including a drafttube disposed centrally in each of said vessels and an agitatorpositioned axially in each of said first slurry from said first reactionvessel to said second reaction vessel, a second conduit interconnectingsaid vessels for conducting said second slurry from said second reactionvessel to said first reaction vessel, an inlet pipe for introducingphosphate rock and phosphoric acid to said first reaction vessel, andwherein a vent is connected to said inlet pipe to permit escape of gasesand, thus, increasing the rate of flow of said phosphate rock andphosphoric acid to said first reaction vessel.

The invention also includes a system for the preparation of phosphoricacid from phosphate rock and sulfuric acid, including in combination:

a. A first reaction vessel containing a first slurry comprising calciumsulfate hemihydrate, monocalcium phosphate and phosphoric acid.

b. A second reaction vessel containing a second slurry comprisingcalcium sulfate hemihydrate, monocalcium, phosphate, sulfuric acid andphosphoric acid.

c. Means in each of said vessels for maintaining a continuouscirculation of the slurry therein from the bottom to the top of eachvessel and from the top to the bottom of said vessel.

d. A first conduit interconnecting said first and second reactionvessels for conducting said first slurry from said first reaction vesselto said second reaction vessel.

e. A second conduit interconnecting said vessels for conducting saidsecond slurry from said second reaction vessel to said first reactionvessel,

f. Means for applying a vacuum to said second reaction vessel to effecttemperature control in said second reaction vessel and to form a vacuumseal between said first and second reaction vessels,

g. Means for introducing phosphate rock and phosphoric acid to saidfirst reaction vessel, and including venting means to remove gases fromsaid introducing means,

h. Means for introducing sulfuric acid to said second reaction vessel,

i. Means for withdrawing a slurry containing phosphoric and calciumsulfate hemihydrate from said second reaction vessel.

Preferably the system also includes a third reslurry vessel forreslurrying phosphate rock and recycle phosphoric acid, and a thirdconduit interconnecting said third vessel with said means forintroducing phosphate rock and phosphoric acid into said first reactionvessel.

Also preferred is that said means for maintaining a continuouscirculation of the slurry in each of said first and second reactionvessels includes a draft tube disposed centrally in each of said vesselsand an agitator positioned axially in each of said vessels within saiddraft tube, whereby on actuation of said agitator the slurry in each ofsaid vessels will flow from the bottom portion of said draft tube upthrough the draft tube and on exiting the top of the draft tube, theslurry will flow downwardly in the space between said draft tube and theinner wall of the vessel.

In the system, said first vessel can be a dissolver vessel foressentially dissolving phosphate rock in said first slurry, said secondevacuated reaction vessel being cooled by evaporation and functioning asa crystallizer vessel for crystallizing calcium sulfate hemihydrate insaid second slurry, and including a fourth, filter feed, vessel, and afourth conduit interconnecting said means for withdrawing slurry fromsaid second reaction vessel with said fourth vessel, for conducting saidsecond slurry containing crystallized calcium sulfate hemihydrate andphosphoric acid to said fourth vessel, and an agitator in said fourthfilter feed vessel for maintaining the slurry therein in suspension.

The fourth circuit can include a surge tank, filter means and,preferably there is a rock box in said conduit for removing anyrelatively large rocks in said second slurry.

The system can contain a fifth conduit for conducting slurry containingcrystalline calcium sulfate hemihydrate and phosphoric acid from saidfourth filter feed vessel to said filter means, for filteringcrystalline calcium sulfate hemihydrate from said slurry, and a sixthconduit connecting said filter with said third reslurry vessel forconducting filtrate containing phophoric acid to said third vessel.

Preferably said second reaction vessel is positioned at an elevationhigher than said first reaction vessel, said second conduit being anoverflow conduit permitting return of said second slurry in said secondvessel by gravity through said second conduit to said first slurry insaid first vessel.

In the system, said means for introducing phosphoric acid and phosphoricacid into said first vessel can comprise an inlet pipe connected withthe interior of said draft tube, and a vent connected to said inlet pipeto permit escape of gases and reduce foaming generated by the dissolvingreaction in said first vessel.

The system can include a sparger in the bottom portion of said secondvessel below the draft tube therein, said means for introducing sulfuricacid into said second vessel comprising an inlet, said inlet beingconnected to said sparger. Preferably the sparger directs the acid tothe bottom of the vessel.

The system preferably includes a first recirculation conduit forselectively recirculating said first slurry from said first vesselexternally thereof and back to said first vessel, a first pump in saidfirst recirculation conduit, a second recirculation conduit forselectively recirculating said second slurry from said second vesselexternally thereof and back to said second vessel, a pump in said secondrecirculation conduit, and valve means for discontinuing slurry flow insaid first conduit from said first vessel to said second vessel duringrecirculation of slurry through said first recirculation conduit or saidsecond recirculation conduit.

Another invention of John Ellis, Michael Gragg and A. Adams, which canbe useful in the present process and system is an apparatus for thepreparation of phophoric acid from phosphate rock and a strong acid,which comprises a closed vessel, a draft tube, means connected to theinner wall of said vessel and mounting said draft tube in a verticalposition within said vessel, said draft tube having an outwardly flaredlower skirt portion terminating in the bottom portion of said vessel, anagitator positioned within said draft tube, a shaft for said agitatormounted axially of said vessel and extending into said draft tube, aninlet conduit to said vessel for introducing a feed slurry of phosphaterock and strong acid into said vessel, said inlet conduit having a lowerend portion terminating within said draft tube, and a vent pipeconnected to said inlet conduit to reduce foaming generated by thereaction in said vessel. This apparatus is claimed in commonly ownedapplication, Ser. No. 866,990, filed on the same day as the subjectapplication. The entire disclosure of said application of Ellis et.,al., is incorporated herein by reference.

In another apparatus, which is useful in the present invention, saidinlet conduit has an elongated portion extending downwardly within saidvessel, said inlet conduit having an external upper portion and an inletportion connected to said upper portion, said feed slurry being fed intosaid inlet portion, and said upper portion of said inlet conduit beingsaid vent pipe.

These features and others of the above described apparatus areillustrated in the accompanying FIG. IV.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. I is a schematic of a preferred embodiment of the process andsystem;

FIG. II shows a schematic of another embodiment which can be used forphosphoric acid manufacture by a gypsum type process;

FIG. III, shows a schematic of a preferred inter- and intra- vessel flowpattern;

FIG. IV, is a cross-section of a dissolver vessel illustrating the useof a vent pipe to reduce foaming and preferred relative positions offeed pipes in relation to the draft tube and agitator.

FIGS. V, VI, and VII, are respectively photomicrograph of the"raspberry", "jacks", and "needles" forms (or crystal habits) ofhemihydrates. FIG. VIII is a photomicrograph of a needle type habitwhich is very small and difficult to filter. FIG. IX illustrates apreferred "stable region" (of temperature and % P₂ O₅, in the calciumsulfate liquid phase) for formation of hemihydrate crystals thecoordinates of the region are about 90° C., 38%; 110° C., 50%; and 90°C., 50%.

BACKGROUND AND FURTHER DESCRIPTION

The invention is especially directed to the production of phosphoricacid by the calcium sulfate hemihydrate process. In the presentinvention, compared to the prior art "gypsum processes", the control ofreactant concentrations and filtration rates are improved, aconcentrated phosphoric acid (about 35% to about 55% P₂ O₅) is produced,sulfuric acid usage is reduced and a substantial reduction in electricaland heat energy consumption is realized.

Phosphoric acid has been prepared by the wet process for many years. Thewet process involves the reaction of phosphatic solid materials,hereinafter termed phosphate rock, wherein calcium sulfate, monocalciumphosphate, phosphoric acid and sulfuric acid comprise the usual reactionmedia. The names of the three processes for the production of phosphoricacid by the wet process are based on the by-product calcium sulfateproduced; namely, the gypsum or dihydrate process, the hemihydrateprocess, and the anhydrite process. In all such processes the presentinvention can be useful in the reaction of phosphoric acid and phosphaterock. The type of by-product is dependent upon the temperature of thesystem and the P₂ O₅ concentration of the liquid phase of the slurry.Other factors such as fluorine concentration, alumina concentration, andsulfuric acid concentration play a less important role.

As is frequently illustrated by composition diagrams (plottingtemperature versus acid strength), gypsum, (CaSO₄,2H₂ O) is theby-product formed when the wet process is run at a temperature of about90° C. or less and a P₂ O₅ concentration of up to about 30% in theliquid portion of the slurry. Increasing the temperature to about 80°-120° C. and the P₂ O₅ concentration to about 40% in the liquid phasewill yield hemihydrate, CaSO₄.1/2H₂ O. Adjusting the temperature and theconcentrations, for instance, to 75° C. and 40% P₂ O₅ results in amixture of gypsum and hemihydrate which is very unstable. An unstablesystem such as this causes trouble during filtration due to thehardening or setting up of the gypsum-hemihydrate solid on the filter.Care must be exercised in maintaining the proper temperature and P₂ O₅concentrations in the process being run in order to avoid such problems.CaSO₄ anhydrite is produced at temperatures of about 130° C. and P₂ O₅concentrations greater than 30%. This latter process is difficult to rundue to severe corrosion at the higher temperatures and the instabilityof the anhydrite during processing.

The stable region in FIG. IX represents a practical region forplant-scale operation of the present invention in a hemihydrate processat a good filtration rate and without encountering problems due toanhydrate or dihydrate formation. FIG. IX also shows conditions whereanhydrite or dihydrate will be produced.

It should also be noted that the use of a sodium organosulfonate crystalmodifier in the process system and appartus herein does not cause anadverse increase in viscosity (contrary to what one would predict fromthe prior art). In fact, with an organosulfonate crystal modifier, theviscosity can be decreased, as is reported by Sikdar and Ore' (one ofthe present applicants) in AIChE JOURNAL (Vol. 23, No. 3), pages380-382, May, 1977.

Mixing is critical to filterability of the hemihydrate and a preferredpropeller blade is of the airfoil or hydrofoil design to reduce shearand vortex formation (e.g. those marketed by Mixco). Thorough mixing isvery desirable, whether running the dihydrate, the hemihydrate or theanhydrite process. Good mixing will decrease the localized highconcentration of the reactants, namely, the calcium phosphate and thesulfuric aacid. Decreasing such localized concentrations, results in alowering of the substitution losses, a lowering of losses due to coatingthe rock and an improvement in the crystallization conditions.

However, if the mixing involves very high shear, the desirable formationof large, "raspberry" hemihydrate may not occur. It is possible that the"raspberry" habit (as illustrated in FIG. V) may be formed by growtharound the needle like projections of the "jacks" habit (illustrated inFIG. VI) and that continued high shear cause these projections to breakoff to form "needles" (as illustrated in FIG. VII). It is also possiblethat improper mixing is a cause for the formation of small fine crystalswhich are very difficult and filter (see FIG. VIII).

Thus, it is observed that a change of one variable may favorably affectthe recovery of P₂ O₅ from phosphate rock employing one of the wetprocess methods and it may be detrimental to the recovery of P₂ O₅employing a different process. Therefore it is necessary to choose thecombination of process variables which will result in the best recoveryof P₂ O₅ from the phosphate rock along with acceptable filter-ability ofthe resulting slurry for the process at hand.

The recovery of the phosphate values from the phosphate rock can begreatly increased if the agitation or mixing is maintained at a highlevel. Previous workers in the field have directed their energy toachieve maximum mixing in the wet process. As a result of this activity,today there are one vessel and multi-vessel systems in use for theproduction of phosphoric acid by wet process. The purpose is to achievemaximum mixing so as to increase the recovery of the phosphate valuesfrom the phosphate rock and to have the best environment for dissolutionof the rock and for crystallization of CaSO₄.

In a one vessel process, the phosphate rock and the sulfuric acid areadded to the slurry in one tank. Agitators, in union with baffles, areused to circulate the slurry into which the reactants (phosphate rockand sulfuric acid) are added. To the extent that the localizedconcentration differences are minimized, the slurry has only one sulfatelevel. This is undesirable, since the phosphate rock should preferablybe dissolved at a low sulfate concentration (preferably negative)whereas crystallization should occur at a high sulfate concentration.

A multi-vessel system can be of two types. Two or more compartments orcells can be constructed within one vessel, the compartments beinginterconnected in series. The reactants are added separately, that is,in different compartments in order to increase the dispersion of saidreactant in the slurry prior to reacting with the other reactant. At thelast compartment, some slurry is removed from the system for recovery ofphosphoric acid; the major portion of the slurry being recycled to thefirst compartment.

Multi-vessel processes involve the use of two or more vessels connectedin series, the reactants are added to the slurry in separate vessels soas to more completely disperse one reactant in the slurry before it iscontacted by the later added reactant(s). Again the system is arrangedso that a portion of the slurry is recycled from a later reactor back tothe first reactor.

The reaction between sulfuric acid and phosphate rock is exothermic. Inorder to control the temperature of the reaction system, provisions mustbe made to remove this excess heat. Previously this has beenaccomplished by (1) blowing air through the slurry or (2) pumping aportion of the slurry to a vessel under vacuum or (3) conducting theoperation in a vessel under vacuum.

The use of air as a coolant is not too desirable because it is necessaryto scrub large amounts of air exiting the system to remove pollutants,mainly fluorine in the form of hydrogen fluoride or silicontetrafluoride. The equipment required is quite expensive.

When a portion of the hot slurry is removed from the main body of theslurry, and subjected to vacuum, cooling occurs by the evaporation ofwater (U.S. Pat. No. 2,699,985). The cooled slurry is then added to themain body of the hot slurry and moderates the temperature of theprocess.

The method of conducting the reaction under vacuum has many desirablefeatures. The cooled slurry is immediately dispersed within the hotslurry and temperature differentials within the slurry are minimized.The slurry is concentrated by the removal of water, and the desiredtemperature is easily maintained.

The above described multi-compartment and multi-vessel systems improvedon dispersing the reactants to some extent, however, greater dispersionof the reactants is desirable in order to improve the dispersion of thereactants in a one vessel reactor. Caldwell, U.S. Pat. Nos 3,416,889 and3,939,248 and Bergstrom, U.S. Pat. Nos. 3,666,143 and 3,917,457developed a combination reactor-cooler which is fitted with a drafttube. The vessel was maintained under a vacuum while the slurry wascirculated within a vessel. Using the draft tube with an agitator it ispossible to circulate the slurry at such a flow rate that upwards of200% of the volume of the slurry is circulated through the draft tubeper minute, constantly renewing the surface of the slurry exposed to thevacuum. With this type of circulation, dispersion of the reactants isimproved over the conventional one vessel system. In addition to betterdispersion of the reactants, the slurry on exposure of the vacuum at thesurface is cooled by evaporation of water. The temperature differentialwithin the system is minimized by the rapid flow rate realized. Thecooled slurry is immediatley mixed with the hot slurry minimizing thelocalized cooling affect.

Lopker, U.S. Pat. Nos. 3,522,003 and 3,522,004 describes the use in agypsum process of a two vessel system for the production of phosphoricacid from phosphate rock and sulfuric acid. A vacuum is applied to onevessel and cools the slurry by evaporation of water. The cooled slurryis then rapidly dispersed within the system minimizing cooling effectsand preventing supersaturation of the calcium sulfate due to reducedtemperatures. The levels of the slurries within the two vessels arevertically offset.

Sulfuric acid, phosphoric acid, phosphate rock or a mixture ofphosphoric acid-phosphate rock can be added to the slurry in differentvesels. The reactants are mixed in the vessel and are circulated fromone vessel to another. In this way, localized, high concentations of theadded reactants are minimized. Good recovery of P₂ O₅ values from therock are realized. Better filtration rates are also obtainable due tothe retardation of the formation of excessive number of very smallcalcium sulfate crystals resulting from supersaturation.

Processes for the production of phosphoric acid by the hemihydrateprocess are well known in the art. A. V. Slack, in "Phosphoric Acid"Part One, Marcel Dekker, Inc., New York, 1968, describes hemihydrateprocess. The problems encountered are observed in filtering thehemihydrate slurry and the high degree of substitution of phosphate inthe calcium sulfate lattice. Attempts to overcome the deficiency infiltration rate and poor P₂ O₅ recoveries while maintaining theproduction of phosphoric acid containing about 40% P₂ O₅, resulted inthe development of a hemihydrate-dihydrate system. U.S. Pat. Nos.3,472,619 and 3,552,918 are representative of the systems employed.

These patents describe the preparation of phosphoric acid by thehemihydrate process, recovering said phosphoric acid from the solidCaSO₄.1/2H₂ O, recrystallization of CaSO₄.1/2H₂ O to CaSO₄.2H₂ O, andthe recovery of phosphoric acid liberated during the recrystallizationof CaSO₄.2H₂ O. Apparently, the best of both processes is achieved. Highconcentration, about 40% P₂ O₅ acid is recovered while low losses in thefilter cake are observed as the result of the recrystallization of theCaSO₄.1/2H₂ O.

Fitch (U.S. Pat. No. 3,552,918) describes a process for the productionof concentrated phosphoric acid and gypsum including the acidulation ofphosphate rock in a first zone in which the resulting slurry containsfrom about 1% (-2.45% SO₄ ⁻⁻) to about 4.5% (-11% SO₄ ⁻⁻) excesscalcium. The slurry produced in the first zone is then transferred to asecond zone in which an excess of sulfuric acid is present such thatfrom about 3% to about 6% excess sulfuric acid is present in the slurry.Hemihydrate initially produced is converted to gypsum.

Peet (U.S. Pat. No. 2,885,263) describes an anhydrite process and Long(U.S. Pat. No. 3,453,076), Peet (U.S. Pat. No. 2,885,264) and Robinson(U.S. Pat. No. 3,418,077) describe processes for the production ofphosphoric acid by the hemihydrate process. No additionalrecrystallization of the CaSO₄.1/2H₂ O is required in these hemihydrateprocesses. In the Robinson process phosphoric acid containing from about40% to about 55% P₂ O₅ by weight is produced.

This process comprises in a first stage reacting in the presence ofexcess calcium ions, phosphate rock with at least nine parts by weightof phosphoric acid for each part of calcium added, said phosphoric acidcontaining at least 37% by weight P₂ O₅ and 1% to 3% by weight dissolvedsulfate, whereby the phosphate rock is converted into a slurrycomprising monocalcium phosphate, phosphoric acid, and calcium sulfate,the percentage of calcium ion precipitated as calcium sulfate being 10to 60%, preferably 20-50% by weight of total calcium fed, in a secondstage reacting the slurry from the first stage with sulfuric acidwhereby phosphoric acid containing at least 40% P₂ O₅ by weight andcalcium sulfate hemihydrate is formed, the sulfuric acid being used inan amount 0.5 to 2.0% by weight in excess of that required to convertthe calcium content of the phosphate rock fed to the first stage intocalcium sulfate. In the third stage, the phosphoric acid is seperatedfrom the calcium sulfate and the crystals are washed. The temperature ofthe first and second stages are in the range from 80° to 111° C.,preferably from 90°-110° C.

DETAILED DESCRIPTION

This invention is directed to a process for the production of phosphoricacid wherein phosphate rock is reacted or "dissolved" in phosphoric acid(but can be useful where nitric acid is used).

A major element of the present invention is that the stream or slurry(comprising phosphoric acid and calcium sulphate hemihydrate) which ispassed through the separation section (e.g. a filter), has abeneficially low content of fines (i.e. solids with an average particlediameter less than about 5 microns) and a low viscosity, especially whenan organic sulfuric acid or organic sulfuric acid salt is added as acrystal modifier at a concentration in the range of 1-1,000 ppm, morepreferred 5-100 ppm, by weight based on the total weight of the slurrytransferred to the separation section.

For production of phosphoric acid in a large scale commercial plant, itis essential that the stream to the filter have a high filtration rate,preferably at least 0.2 tons P₂ O₅ /day ft² (more preferred at least0.5). Typically, the stream to the filter in the present process has afiltration rate of at least 0.6 tons P₂ O₅ /day-ft² and has about atleast as fast a filtration rate as does the usual stream to the filterfrom a Prayon type gypsum process utilizing the same rock, water,sulfuric acid, etc. (yet at a significantly higher P₂ O₅ concentrationin the product acid, e.g. 28% for gypsum versus 42% for thehemihydrate).

The filter rate can be calculated as shown in the applications of Ore etal, filed Dec. 29, 1977.

Another very important factor is that the hemihydrate crystals in thefeed to the filter in the present process has a fairly uniform particlesize distribution and are in the "raspberry" crystal habit or form andhave a very low fine content (as illustrated by the photomicrograph inthe accompanying FIG. V).

FIG. VI is a photomicrograph of a more usual (especially where there areno impurities in the environment e.g., Fe Al, Mg, fluoride, SiF₄ ⁻²)"jacks" type of calcium sulfate hemihydrate crystal habit (which may bea precursor of the raspberry crystal).

FIG. VII shows a "needle" form of calcium sulfate hemihydrate crystalhabit. The FIG. VII crystals may possibly be formed by fission from the"jacks" type habit (e.g. by high shear agitation).

FIG. VIII is a photomicrograph of a very poor small needle type crystalhabit which is very difficult to filter due to the high proportion offines.

It should be noted that when operating the invention as described hereinfiltration has been very good; however, on some occasions solids havebuilt-up and caused buckling of the filter elements, probably due toloss of sulfate control.

Phosphate rock, either calcined or uncalcined, and phosphoric acid areadded to a first slurry comprising phosphate rock, calcium sulfatehemihydrate, monocalcium phosphate, phosphoric acid and sulfuric acid.Preferably, the phosphate rock is slurried in the phosphoric acid priorto the addition to the first slurry. Phosphate rock, about 95% of +100mesh, containing at least 32% P₂ O₅ is the preferred source of phosphatefor the process. Ground or unground rock can be used. For example,phosphate rock of 95% of -200 mesh can be used. Rock containing lessthan 32% P₂ O₅ is acceptable, and can be employed in this process. Highalumina phosphate pebble may also be used, especially when the resultingacid is purified by the process of U.S. Application Ser. No. 676,559filed Apr. 13, 1976 of Ore, the entire disclosure of which is herebyincorporated herein. The phosphate rock is slurried in phosphoric acidthat contains from about 20% to about 40% P₂ O₅. Phosphoric acid,recycled from the separation section, containing from about 20% to about40% P₂ O₅ (and usually some sulfuric acid) is usually used in theprocess. When the phosphoric acid is recycled from the separationsection it will usually contain from about 0.5% to about 3.5% sulfuricacid by weight. Phosphoric acid from other sources, such as otherphosphate plants, merchant grade acid may be used.

The temperature of the phosphate rock-phosphoric acid mixture ismaintained at about 50° C. to about 100° C., preferably from about 90°C. to about 100° C. The resulting mixture is from about 30% to about 40%solids by weight, about 33% being preferred. A defoamer is added if andwhen required. Calcination of the rock can reduce or eliminate foaming.Various antifoam agents can be used, including tall oil, tall oil fattyacids, alkyl esters and part esters of tall oil fatty acids, sulfatedtall oil fatty acids, tall oil rosin, alkoxide adducted tall oil rosin,oleic acid, sulfated oleic acid, silicones, reaction products of aminesand carboxylic acids and mixtures of two or more of such defoamers.

The phosphate rock-phosphoric acid mixture is added to a first slurry ofcalcium sulfate hemihydrate, phosphoric acid, monocalcium phosphate andsulfuric acid in a first reaction vessel. The phosphate rock andphosphoric acid may be admixed in a separate vessel or added separatelyto the first slurry in the first reaction vessel. The phosphaterock-phosphoric acid mixture on being added to the first slurry in thefirst reaction vessel is rapidly dispersed within the first slurry. Afirst portion of the first slurry is transferred to a second reactionvessel.

The first reaction vessel is fitted with a draft tube and an agitator(although the draft tube can, in some cases, be removed). The agitatorcan consist of a shaft fitted with a propeller at the bottom thereof.The agitator is so located with respect to the draft tube that onactivation of the agitator, a second portion of the first slurry isdrawn from the bottom of the draft tube up through the draft tube andout the top of the draft tube. On exiting the draft tube said slurrypasses in a downward direction in the space between the draft tube andthe walls of the first reaction vessel. The direction of circulationthrough the draft may be reversed and is not critical. In this first ordissolver vessel (or set of vessels), considerable gas (CO₂) isgenerated and, like a giant milkshake, the apparent density of thecontents can be about 1.0, although when the gas is removed, the actualdensity of the contents is about 1.6 to 2.0.

Circulation is thus established within the first reaction vessel. Therate at which said slurry is circulated is at least equal about 50% ofthe volume of the slurry in the first reaction vessel per minute,preferably from about 50% to about 150% and most preferably about 100%.This circulation thoroughly disperses the phosphate rock-phosphoric acidmixture within the first slurry. The recycle phophoric acid dissolvesthe P₂ O₅ in the rock forming monocalcium phosphate. This is anexothermic reaction which supplies the heat required to maintain thetemperature of the slurry in the first reaction vessel between about 66°C. to about 113° C. The soluble or "excess" sulfate content of the firstslurry is typically maintained at about +0.0% to about -4%, (but,depending on the method of sulfate analysis, the soluble sufate can beas low as -7%, below which the rock dissolution practically stops due tosaturation of calcium phosphate). The first slurry, whether in one or aplurality of reactors is maintained at an excess of Ca⁺² (a deficiencyof SO₄ ⁻²) for stoichiometric formation of CaSO₄.

The temperature and P₂ O₅ content in the dissolver vessel and especiallyin the crystallizer or second vessel, are preferably within the regionshown on the striped rectangle labeled "stable region" in theaccompanying FIG. IX, titled "Hydrates of CaSO₄ Precipitated VS % P₂ O₅and Temperature".

As measured, soluble sulfate values can be either positive or negative.Soluble sulfate values include not only the sulfuric acid present in theliquid component of the slurry but also the soluble calcium sulfatethere present. Negative soluble sulfate values indicate that excess ofcalcium ions are present in the solution, as is usually observed in thephosphate rock-phosphoric acid mixture. Positive soluble sulfate valuesindicate that excess sulfate ions are present. A value of 0.0% indicatesthat the sulfate ions and the calcium ions are equivalentstoichiometrically within the limits of the analysis.

One typical analysis is 0.9% CaO and 2.2% SO₄ which would calculate##EQU1##

This leaves 2.2-1.5=+0.7% "free or soluble" sulphate.

That is, a positive soluble sulphate.

Another analysis is 0.98% CaO and 1.4% SO₄, which calculates ##EQU2##That is, there is insufficient sulphate concentration to combine withall of the calcium, which is reported as a negative value.

As is described further in the applications of Ore et al, filed Dec. 29,1977, the numerical value of negative sulfate can vary somewhatdepending on the analytical procedure for sulfate ions.

For positive sulfate values, there is little or no difference betweenvalues obtained by different analytical methods. The preferred method ofcalcium analysis is by atomic absorption, which is highly accurate forboth positive and negative sulfate.

The residence time of the solids in the first reaction vessel is fromabout 2.0 hours to about 5.0 hours, preferably from about 2.5 hours toabout 4.5 hours.

A first portion of the first slurry is transferred through a firstconduit into a second reaction vessel. The second reaction vessel, whichis preferably subjected to a vacuum, is fitted with a draft tube, anagitator and a sulfuric acid inlet. The agitator consists of a shaftfitted with a propeller at the bottom thereof. The shaft and agitatorare so located with respect to the draft tube that on actuation of theagitator a second portion of the second slurry is caused to flow fromthe bottom of the draft tube up through the draft tube and out the topof the draft tube. On exiting the draft tube, said second portion of thesecond slurry flows in a downward direction in a space between the drafttube and the inside walls of the second reaction vessel. The directionof the circulation can be reversed and is not critical. The rate atwhich the slurry is circulated is at least equal to about 50% of thevolume of the slurry in the vessel per minute, preferably from about 50%to about 150% of the volume and most preferably about 100 % of thevolume.

Sulfuric acid, preferably about 98%, is added through the sulfuric acidinlet into the second slurry either as is or mixed with phosphoric acid.The first portion of the first slurry is also added into the secondslurry.

A crystal modifier, usually a derivative of tall oil or of an organicsulfonic acid, preferably a salt, can be added to the slurry in thesecond reaction vessel. The crystal modifier can also be added to thefirst reaction vessel. A preferred crystal modifier is selected fromalkyl, aryl, alkylaryl, and alicyclic derivates of sulfonic and sulfuricacids in which the organic radical contains from about 12 to about 30carbon atoms. The free acid, salts thereof and mixtures of the free acidand salts may be used. The preferred salts include those of alkalimetals, ammonia and alkyl, aryl or alkylaryl amines (e.g. trimethylamine, diethyl amine, monopropyl amine). Polymeric sulfonates andsulfates can also be employed. Examples of crystal modifiers which canbe employed in the present process are alkyl sulfonic acids containingfrom about 12 to about 30 carbon atoms, benzenesulfonic acid,alkylbenzenesulfonic acid in which the alkyl group contains from about 8to 20 carbon atoms, alkylcyclohexane sulfonic acid in which the alkylgroup contains from about 8 to 20 carbon atoms, polymeric sulfonates andsulfates such as polystyrene sulfonate, and polyvinylsulfonate, saidpolymeric material having a molecular weight of from about 500 to about1,000,000. The organic sulfonic acid can be an alkyl-,aryl-,or analkylaryl-sulfonic acid or a sulfated derivative of a carboxylic acid oran alkalimetal, amine or ammonium salt thereof.

For example, tetradecylsulfonic acid, benzene-sulfonic acid,isooctylbenzene sulfonic acid and sulfated oleic acid may be used ascrystal modifiers in this process. Mixtures of two or more modifiers arealso useful. The crystal modifier is added for the purpose of increasingthe growth of the hemihydrate crystals formed in the system. Thepreferred salts include those of sodium, potassium, ammonia and primary,secondary and tertiary alkyl amines containing from 1 to about 30 carbonatoms. Preferably, the modifier as described above, is present at alevel of about 1 to 1,000 ppm (usually 5 to 500 ppm) based on the weightof slurry to the separation section. Preferably, the levels of modifierand of defoamer are kept as low as possible (while maintaining goodfilterability), since residual quantities in the phosphoric acid productcan cause crud formation (e.g., emulsions and deposits) if the acid islater treated to remove magnesium impurities by the process of U.S.Patent application Ser. No. 688,265 filed May 20, 1976 issued Oct. 11,1977 as U.S. Pat. No. 4,053,564 (which is incorporated herein) and therelated processes in U.S. Ser. No. 840,791 filed Oct. 11, 1977.

An unexpected discovery is that salts of sulfonic acids can be used ascrystal modifiers in the process of the present invention withoutcausing an adverse increase in solution viscosity. This result issurprising, in view of the teaching in Slack, Phosphoric Acid, M.Deckker, Inc., New York, 1968 (at pate 383), "Neutral surfactants, suchas a salt of ABS, have an adverse effect on slurry viscosity".

The flow of the second slurry within the second reaction vesselthoroughly disperses the first portion of the first slurry, the sulfuricacid and the crystal modifier within the second slurry. The location ofthe sulfuric acid inlet in the second reaction vessel is not critical.It may be at the top, the middle, the bottom or at intermediate pointsof the second reaction vessel. The sulfuric acid conduit attached to thesulfuric acid inlet may enter the second reaction vessel from the top,the bottom, or points intermediate therein, the exact point of entranceinto the vessel is not critical.

Phosphoric acid, if needed, can be added to the second slurry within thesecond reaction vessel.

The surface of the second slurry in the second reaction vessel ispreferably exposed to a pressure of between about 2 to about 29 inchesof mercury absolute, more preferably from about 3 to about 20 inchesmercury absolute. Water and volatile components added to, or producedin, both the first and second slurries can be removed from the secondslurry by evaporation causing a reduction in the temperature of thesecond slurry. The cooled second slurry is thoroughly mixed so thattemperature differentials are minimized within the total volume of thesecond slurry. With this evaporative cooling, the temperature of thesecond slurry is maintained between about 66° C. to about 113° C.preferably from 80° C. to about 105° C. Although it is greatlypreferable to operate the second vessel under reduced pressure, theprocess can be run while maintaining both the first and second reactionvessels at atmospheric pressure. Sulfuric acid, which is added to thesecond slurry in the second reaction vessel through the sulfuric acidinlet, can be from about 89% to 99% H₂ SO₄, typically about 98% H₂ SO₄.

The total sulfate value added to the system is the sum of the sulfatevalues in sulfuric acid added plus the sulfate values added in the rock.Surprisingly in the present process this total can be only about 90% to100% of the stoichiometric amount of sulfate needed to convert thecalcium added in the rock fed to the first reaction vessel into calciumsulfate hemihydrate. Table 1 herein is a compilation of such sulfuricacid usage. Listed are the tons per day (TPD) of phosphate rock fed, %CaO in the rock, % SO₄ ⁻⁻ in the rock, CaO fed (TPD), stoichiometricsulfate for the calcium in the rock (TPD), sulfate in sulfuric acid fedto the unit (TPD), sulfate equivalent in the rock (TPD), the totalsulfate used (TPD), and total sulfate used as a fraction of thestoichiometric amount of sulfate required for the calcium in the rock.

The soluble sulfate content as measured in the second slurry should befrom about +0.7% to about +4.5%, preferably from about 1.5% to about3.0%. The specific gravity of the slurry in the second reaction vesselis about 1.80%±.2 g/cc. The specific gravity of the liquid portion ofthe slurry is about 1.56±0.20 g/cc. The liquid gravity corresponds to aphosphoric acid which contains about 44%±10% P₂ O₅. Residence time ofthe solids in the second reaction vessel is from about 0.6 hour to about2.0 hours, preferably from about 0.7 hour to about 1.6 hours.

The excellent mixing obtained with this system is achieved usingapproximately 1/16 of the horsepower required for a comparable wetprocess gypsum type phosphoric acid plant such as a Dorr-Oliver or aPrayon Plant.

A first portion of the second slurry flows from the second reactionvessel back to the first reaction vessel through a second conduit and isthoroughly dispersed within the first slurry.

It is the flow of the second slurry to the first slurry which aids incontrolling the temperature of the first slurry and adds sulfate values(sulfuric acid) and phosphoric acid values to the first slurry in orderto dissolve the rock. Additional sulfate values are usually added to thefirst slurry in the first reaction vessel with the recycled phosphoricacid. Circulation between vessels and within vessel minimizes localizedconcentration of reactants of hot slurry and of cooled slurry thusresulting in a more easily controlled process than previously observed.

A third portion of the second slurry is removed from the second reactionvessel and is transferred through a conduit to a reservoir. The thirdportion of the second slurry, on a weight basis, is approximately equalto the phosphate rock, the phosphoric acid, and the sulfuric acid addedin the first and second reaction vessels respectively minus thevolatiles (on a weight basis) removed from the second reaction vesselwhich can be subject to a vacuum.

For plant control purposes, the flow rates of the reactants and of theslurries can be adjusted in accordance with the analytical valuesobtained in order to maintain the desired sulfate levels within thereaction system. It is to be understood that the system described can berun on a batch or continuous basis (wherein the reactants can becontinuously added and the third portion of the second slurry can becontinuously removed from the system prior to separation into phosphoricacid and calcium sulfate hemihydrate).

A defoamer is added if and when required. The defoamer may be selectedfrom the group consisting of tall oil rosin, alkoxylated tall oil rosin(see U.S. Pat. No. 3,594,123, issued July 20, 1971 to Encka et al), talloil fatty acids, whole or part esters of tall oil fatty acids, sulfatedtall oil fatty acids, alkoxylated tall oil fatty acids, oleic acid,sulfated oleic acid, silicones and mixtures of a monocarboxylic acid(12-22 carbon atoms) and monoalkanoylamide derivatives of themonocarboxylic acid. The preferred defoamer is a mixture of methylesters of tall oil fatty acid and tall oil fatty acids sold by AZProducts Co. of Eaton Park, Fla. under the tradename "AZ 10A" (becauseAZ 10A also acts as a crystal modifier). The crystal modification is notdue entirely to the presence in AZ 10A of a sodium alkyl sulfonatebecause crystal modification also occurs when the sulfonate is removed.The amount of the defoamer used is preferably from about 0.01% to about0.3% (typically 0.04 to 0.1) by weight based on the weight of the slurrytransferred to the separation section (or about 0.05% to 1.5% based onP₂ O₅ produced by the process). As is noted hereinafter, venting of thereslurry and/or dissolver vessels can reduce defoamer usage.

AZ 10A is further discussed in the applications of Ore et al, filed Dec.29, 1977.

One of the preferred crystal modifiers, especially when AZ 10A is alsopresent, is Actrasol W-40. Actrasol W-40 is a mixture of predominantlysaturated sodium alkyl sulfonates. The alkyl groups are in the 12 carbonranges, although there is a distribution from about 9-15 carbons. Thereare approximately 16 different sulfonates in the mixture; undoubtedlymany are isomers and homologs of each other. It appears that ActrasolW-40 is made by the sulfonation of propylene tetramer, butylene trimer,or other material consisting of a mixture of isomers and homologs.Actrasol W-40 is further described in the application of Ore et al filedDec. 29, 1977.

The P₂ O₅ yield of the process can be improved by converting thehemihydrate filter cake to dihydrate cake by repulping it in water andsulfuric acid. The dihydrate slurry is filtered to separate the aqueousphase (phosphoric acid 30-35% P₂ O₅) which is recycled to rockdigestion.

There are three analytical procedures which can be used to determinesulfate in phosphoric acid; namely, gravimetric, titration andturbidity. These are all further described in the applications of Ore etal filed Dec. 29, 1977.

With some solutions, where negative sulfate ion is concerned, thetitration method can give higher values than the turbidity method (e.g.,-1.0% sulfate by titration and -2% by turbidity) or -3.3% by titrationversus -6% by turbidity). However, both methods are usually in closeagreement where positive sulfate is determined.

In the present invention the most important factor in the preferredoperation is that by about every analytical method, a negative sulfatebe maintained in the first (dissolver) vessel. Even a slight positivesulfate (e.g. +0.7%) in the dissolver can cause decreases in yield ofphosphoric acid produced by the process (one cause being due to greatlyincreased nucleation, another at about 1.5% SO₄ to coating of thephosphate rock which decreases the amount dissolved).

Mesh size of the rock also influences yield, the smaller the particlesize, the better the yield. Especially good results are obtained by wetscreening rock to -28 mesh or smaller. No drying is needed.

If the second (crystallizer) reactor is not subjected to reducedpressure to provide cooling, external cooling means (such as a flashcooler) can be provided to cool the slurry.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. I, II, and III are schematic diagrams of embodiments of theprocess. In FIG. III, Phosphoric acid at about 70° C., which is addedthrough conduit 6, and phosphate rock, which is added through conduit 8,are slurried in vessel 2 which is fitted with an agitator 4. Defoamercan be added as needed through conduit 10. In a preferred embodiment,conduit 10 is of a much greater height and/or diameter than is requiredsolely for introduction of the defoamer and, thus, can function as avent. Properly chosen venting can greatly reduce foaming and, in manycases, can eliminate the need for a defoamer in the "reslurry" vessel 2.The temperature of slurry 11 so formed is about 92° C. and the solidscontent is about 30% to about 40% by weight. Slurry 11 is transferredthrough conduit 12 to vessel 16. Vessel 16 is fitted with an agitator(shaft 18 and propeller 21 attacted to the bottom thereof), and a drafttube 20 which is secured to the inside wall of vessel 16 by braces (notshown). Slurry 11 flows into slurry 22 which is composed of calciumsulfate hemihydrate, monocalcium phosphate, phosphoric acid, andsulfuric acid. The propeller 21 of the agitator is so positioned withrespect to the location of the draft tube 20 that on actuation of theshaft 18 and propeller 21 by a motor (not shown), slurry 22 in vessel 16will flow from the bottom portion of the draft tube 20 up through thedraft tube. On exiting the top of the draft tube, slurry 22 will flowdownwardly in the space between the draft tube 20 and the inside wallsof vessel 16. Alternatively, the propeller blade can be positionedcloser to the bottom of the draft tube and, if desired, the flow can bereversed (i.e., to flow upwardly in the space between the draft tube andthe inside walls). A first portion of slurry 33 is transferred fromvessel 28 through conduit 38 to vessel 16. The flow created withinvessel 16 thoroughly mixes slurry 11 and slurry 33 within slurry 22.Slurry 22 is then transferred to vessel 28 through conduits 24 usingpump 25. Vessel 28 may be vertically offset from vessel 16 or it may beon the same level as vessel 16. Samples for analysis of the first slurryare removed from sample port 24a. Slurry 22 is at a temperature of about66° C. to about 113° C., and has a soluble sulfate value of about +0.7to about -4% (most preferred below 0.0).

On entering vessel 28 which is equipped with an agitator (shaft 30 andpropeller 31 attached to the bottom thereof), a draft tube 32 and asulfuric acid inlet 34, slurry 22 is dispersed into slurry 33. Drafttube 32 is secured to the inside wall of vessel 28 by braces (notshown). Sulfuric acid is added from the sulfuric acid inlet 34 and isalso thoroughly dispersed into slurry 33. Crystal modifier may be addedto vessel 28 through an inlet 23a. Activation of the agitator (shaft 30and propeller 31) by means of a motor (not shown) causes a flow ofslurry 33 from the bottom of the draft tube 32 up through the draft tubeand out the top portion of said draft tube. On exiting the top of thedraft tube 32, the slurry flows downwardly in the space between thedraft tube 32 and the inside walls of vessel 28. A circulationestablished within vessel 28 disperses slurry 22 and sulfuric acid intoslurry 33, constantly renewing surface 36. Vessel 28 is maintained at apressure of about 2 inches of mercury to about 29 inches of mercuryabsolute. Water is evaporated from the hot slurry, thus cooling theslurry. In addition to water, other volatile materials produced by thereaction of sulfuric acid and phosphate rock are also removed. Thesematerials include CO₂, HF, SiF₄, H₂ S, SO₂ and others. Because of theinternal circulation of the slurry within vessel 28, temperaturegradients are minimized. Slurry 33 (maintained at a temperature of about66° C. to about 113° C., preferably from about 80° C. to about 105° C.,and having a positive sulfate content, preferably about +0.7 to about+4.5%) is recirculated back to vessel 16 through conduit 38. Slurry 33is efficiently dispersed within slurry 22 in vessel 16 by means of theinternal circulation within vessel 16. Thus, a system has been developedin which both inter and intra-vessel circulation occur so as to betterdisperse the reactants being added to the slurries and to reducetemperature gradients within the vessels due to heating and cooling.This circulation system permits rapid and easy plant control and thenegative sulfate level in the dissolver acts like a buffer to adsorbinadvertent sulfate concentration increases in the crystallizer.

A portion of slurry 33 about equal to the amount of reactants added(phosphoric acid, phosphate rock and sulfuric acid), minus the amount ofwater and volatiles removed from the system is removed from vessel 28through conduit 40. Samples for analysis of the second slurry areremoved from sample port 42 located on conduit 40. The slurry is pumped(pump 35) to the diversion or splitter box 44 from which it flows tovessel 48 through conduit 46. Agitator 50 maintains the slurry in adispersed condition in vessel 48. The slurry is pumped (pump 3a) fromvessel 48 through conduit 52 to the separation section (not shown inFIG. III but shown in FIG. I). In start-up, valve 52a can be used todirect the slurry through line 3 to vessel 2, and, thus, bypass thefilter and provide a fast means of building up the solid level in theslurry without dumping wet pans.

Reactants are continuously added to vessel 16 and 28, while water andvolatiles and the product slurry are constantly being withdrawn fromvessel 28. In case of a separation apparatus breakdown, the system canbe placed on recycle. No reactants are added to the system. Intra-vesselcirculation would continue and inter vessel circulation would bediscontinued. As is discussed further hereinafter, the circulationsystem in FIG. III is especially useful in providing an economical andeasy to control start-up or restart (after a short interruption)procedure.

It is to be recognized that the elevation of vessels 2, 16, 28, 44 and48 with respect to each other is a preferred arrangement but may bevaried. Likewise, the conduits connecting vessels 2, 16, 44 and 48 maybe rearranged, additional conduits added and/or existing conduitsdeleted. For example, slurry 22 passing from vessel 16 to vessel 28 maybe introduced into the top part of vessel 28 rather than the bottompart.

Another embodiment of the claimed invention is shown in FIG. II. Insteadof adding the reactants phosphoric acid, phosphate rock and ifnecessary, the defoamer to a preslurry vessel 2 as shown in FIGS. I andIII, the reactants are added directly to the first slurry 22 in vessel16. The phosphoric acid is added through conduit 7 and the phosphaterock is added through conduit 9. The reactants are added in amounts suchthat the direct combination of the two results in a slurry containingbetween about 30% to about 40% solids by weight and an initialconcentration of about 13% to about 47% P₂ O₅ in the liquid portion ofthe slurry. Defoamer is added through conduit 13, if, and when needed.Once the reactants are dispersed in the first slurry 22, the parameterssuch as temperatures, pressures, concentrations, and flows are the sameas described above for the more preferred embodiment.

FIG. II illustrates a less-preferred method of operation useing two feedlines 24 and 26 from the dissolver vessel 16 to the crystallizer 28. Oneline is at least double the diameter of the other. In the hemihydrateprocess described herein, the larger of these lines (and the associatedpump) need not be used. However, the larger (or both lines) can be usedif the system is operated as a gypsum process (as disclosed in Ser. No.703,138). Such double lines can also be incorporated in the systems ofFIGS. I and III if these systems are to be used in a gypsum process.This convertibility from hemihydrate to gypsum is an advantage of thereaction systems described herein.

FIG. III also illustrates a system and apparatus for phosphoric acidproduction by calcium sulfate hemihydrate formation. This system for thepreparation of phosphoric acid from phosphate rock and sulfuric acidincludes in combination a first reaction vessel (the dissolver, 16)containing a first slurry (22) comprising calcium sulfate hemihydrate,monocalcium phosphate and phosphoric acid, a second reaction vessel (thecrystallizer, 28) containing a second slurry (33) comprising calciumsulfate hemihydrate, monocalcium phosphate, sulfuric acid and phosphoricacid, means in each of said vessels for maintaining a continuouscirculation of the slurry therein, said last mentioned means including adraft tube (20 in the dissolver, 32 in the crystallizer) disposedcentrally in each of said vessels and an agitator (18, 30) positionedaxially in each of said vessels within said draft tube, whereby onactuation of said agitator the slurry in each of said vessels will flowfrom the bottom portion of said draft tube up through the draft tube andon exiting the top of the draft tube, the slurry will flow downwardly inthe space between said draft tube and the inner wall of the vessel, afirst conduit (24) interconnecting said first and second reactionvessels for conducting said first slurry from said first reaction vesselto said second reaction vessel, a second conduit (e.g. an overflow 38)interconnecting said vessels for conducting said second slurry from saidsecond reaction vessel to said first reaction vessel, means for applyinga vacuum (as indicated at 28a) to said second reaction vessel to effecttemperature control in said second reaction vessel and to thereby form avacuum seal between said first and second reaction vessels, an inletpipe (12) for introducing phosphate rock and phosphoric acid to saidfirst reaction vessel, said inlet pipe connected with the interior ofthe draft tube in said first vessel, means for introducing sulfuric acidto said second reaction vessel (e.g. a sparger 34) and means (40) forwithdrawing a slurry containing phosphoric acid and calcium sulfatehemihydrate from said second reaction vessel.

The system can include a third reslurry vessel (2) for reslurryingphosphate rock and recycle phosphoric acid, and a third conduit (12)interconnecting said third vessel with said inlet pipe to said firstreaction vessel. In the system, the first vessel can be a dissolvervessel for essentially dissolving phosphate rock in said first slurry,said second evacuated reaction vessel being cooled by evaporation andfunctioning as a crystallizer vessel for crystallizing calcium sulfatehemihydrate in said second slurry, and including a fourth filter feedvessel (48), and a fourth conduit (40b and 46) interconnecting said lastmentioned means (40) for withdrawing slurry from said second reactionvessel with said fourth vessel, for conducting said second slurrycontaining crystallized calcium sulfate hemihydrate and phosphoric acidto said fourth vessel, and an agitator (50) in said fourth filter feedvessel for maintaining the slurry therein in suspension. The system caninclude a surge tank (44) or a splitter box (44a in FIG. 1) in saidfourth conduit which provides a sufficient liquid head to provide avacuum seal and (with the splitter box) a break in the line to preventsiphoning.

A splitter box is a compartmented vessel, usually containing a wire-likepartition (which can be fixed or movable) and is usually used like avalve in a conduit system for a slurry (that is, it usually is used todivert flow).

The system can include filter means (the filter in FIG. 1), a fifthconduit (52) for conducting slurry containing crystalline calciumsulfate hemihydrate and phosphoric acid from said fourth filter feedvessel to said filter means, for filtering crystalline calcium sulfatehemihydrate from said slurry, and a sixth conduit (6) connecting saidfilter means with said third reslurry vessel for conducting filtratecontaining phosphoric acid to said third vessel. The system can includea rock box (see FIG. 1) in said fourth conduit for trapping anyrelatively large rocks, stones and objects (especially solids formed onthe sulfuric acid sparger) in said second slurry. The rock box alsoincludes washing means (e.g., a water line, not shown) and dischargemeans (e.g., a drain, not shown) for removing entrapped particles duringa shut down period or they can be shoveled out. In the system, the firstdissolver vessel (16) can be positioned at an elevation higher than saidfourth filter feed vessel (48) and include an overflow pipe (26) fromsaid first vessel to said fourth vessel for conducting overflow slurryfrom said first vessel by gravity to said fourth vessel.

In the system, the second reaction vessel (28) can be positioned at anelevation higher than said first reaction vessel, said second conduit(38) being an overflow conduit permitting return of said second slurryin said second vessel by gravity through said second conduit to saidfirst slurry in said first vessel. In the system as defined in claim 1,including a vent (5) connected to said inlet pipe (12) to permit escapeof gases and reduce foaming generated by the dissolving reaction in saidfirst vessel. The system can include a sparger (34) in the bottomportion of said second vessel below the draft tube therein, said meansfor introducing sulfuric acid into said second vessel comprising aninlet (34a), said inlet being connected to said sparger. The system caninclude a first recirculation conduit (24, 14) for selectivelyrecirculating said first slurry from said first vessel externallythereof and back to said first vessel, a first pump (25) in said firstrecirculation conduit, a second recirculation conduit (40, 40a, 23b) forselectively recirculating said second slurry from said second vesselexternally thereof and back to said second vessel, a second pump (35) insaid second recirculation conduit, and valve means (15,17) fordiscontinuing slurry flow in said first conduit from said first vesselto said second vessel and to permit recirculation of slurry through saidfirst recirculation conduit or valve means (29,43) to permitrecirculation of slurry through said second recirculation conduit.

FIG. IV shows a preferred apparatus (16) for the preparation ofphosphoric acid from phosphate rock and a strong acid, which comprises aclosed vessel 62, a draft tube 20, means (braces 64) connected to theinner walls of said vessel and mounting said draft tube in a verticalposition within said vessel, an agitator 21 positioned within said drafttube, a shaft 18 for said agitator mounted axially of said vessel andextending into said draft tube, an inlet conduit 12 to said vessel forintroducing a feed slurry of phosphate rock and strong acid into saidvessel, said inlet conduit having a lower end portion 12a terminatingwithin said draft tube 20.

The inlet conduit can have an elongated portion 12b extending downwardlywithin said vessel, and having an external upper vent portion 21 and aninlet portion 70 connected to said upper portion, said feed slurry beingfed into said inlet portion.

In the apparatus, the draft tube can have an outwardly flared lowerskirt portion 66 terminating in the bottom portion of said vessel, andcan include a vent pipe 5 connected to said inlet conduit to reducefoaming generated by the reaction in said vessel. The inlet conduit canhave an essentially vertical portion 12b positioned within said vessel,and have an external vertical upper portion 5 extending above saidvessel and an inlet portion 70 externally of said vessel and connectedat an angle to said vertical external upper portion of said inletconduit, said feed slurry being fed into said inlet portion 70, and saidinterconnected vertical upper portion 1 of said inlet conduit being saidvent pipe.

In the apparatus, the agitator 21 can be positioned in the upper portionof said draft tube 20 (as in FIG. IV), or at the lower portion, or at anintermediate level. The lower end portion 12a of the inlet conduit 12can terminate in the draft tube 20 below the agitator 21. The vessel 16can have an outlet 72 in the lower end thereof below said draft tube 20.

The inner wall 74 of said vessel adjacent to the bottom thereof can bedownwardly dished at 16a adjacent the lower end of said draft tube. Theapparatus can include an overflow pipe 26 extending from the upper endportion of said vessel 62, and a recycle slurry pipe 38 extending intosaid vessel 62 and terminating 38a at a zone in the annulus thereofbetween the outer wall 20 of said draft tube and the inner wall 74 ofsaid vessel, adjacent the lower skirt portion 66 of said draft tube.

The apparatus can include means 1a for introducing a treating ordefoaming agent into said vent pipe 1. In the apparatus, the draft tube20 can have an outwardly flared lower skirt portion 66 terminating inthe bottom portion of said vessel 62, said vessel having an outlet 72 inthe lower end thereof below said draft tube, the inner wall 74 of saidvessel adjacent the bottom thereof being downwardly dished (at 16a)adjacent the lower end 66 of said draft tube 20. The apparatus caninclude an overflow pipe 26 extending from the upper end portion of saidvessel 62, a recycle slurry pipe 38 extending into said vessel andterminating at a zone in the annulus thereof between the outer wall ofsaid draft tube and the inner wall 74 of said vessel, adjacent the lowerend portion of said draft tube, and an additional inlet pipe 68 in saidvessel for the introduction of steam or a treating or defoaming agent.

ILLUSTRATIVE EXAMPLES EXAMPLE 1

Vessels 16 and 28 and the accompanying connective means such asconduits, pumps, etc. of FIG. III are filled with a slurry consisting ofcalcium sulfate hemihydrate, monocalcium phosphate, phosphoric acid andsulfuric acid. The weight percent of the solids in the slurry is about31%, the specific gravity of the degassed slurry in vessel 28 is about1.80 g/cc. P₂ O₅ concentration of the liquid portion of the slurry isabout 42% by weight. The temperature of the slurry in vessel 16 isbetween about 88°-102° C. preferably between 92° C. and 105° C., whereasthe temperature in vessel 28 is between 88° and 105° C., preferably 92°C. and 105° C. Soluble sulfate concentration in vessel 16 is from about+0.6 to about -7% (preferably 0.0 to -6) and the soluble sulfateconcentration in vessel 28 is above 0.0 (preferably from about 0.7% toabout +4.5%).

A mixture of phosphate rock (typical analysis shown in Table 2 and asize distribution shown in Table 3), and phosphoric acid is prepared byadding phosphate rock to phosphoric acid in the ratio of about 1,647pounds of phosphate rock (about 31.2 P₂ O₅ and 45.6 CaO) to about 3,700pounds of phosphoric acid (about 32% P₂ O₅). The temperature of themixture is about 90° C. A tall oil-sulfonic acid defoaming agent (AZ10A) is added as needed to reduce the foam caused by partial dissolutionof the phosphate rock in phosphoric acid.

This phosphate rock-phosphoric acid mixture is added to the first slurryin vessel 16 at the rate of about 380 gpm (about 5,350 pounds perminute). The incoming mixture is thoroughly mixed with the first slurryand a first portion of the second slurry from the second reactionvessel. Intra vessel mixing is accomplished by means of the draft tubeand the agitator. The first slurry is pumped from the first reactionvessel 16 to the second reaction vessel 28 at the rate of about 1,640gallon per minute. The first slurry is thoroughly mixed with the secondslurry and 98% sulfuric acid which is added to the second reactionvessel at about 87 gpm. An organic sulfonic acid derivative can be addedto the second reaction vessel 28. This material is added to promote thegrowth of the hemihydrate crystals. The first slurry, the sulfuric acidand the crystal modifier are thoroughly dispersed into the second slurryin the second reaction vessel 28. The second slurry flows at the rate ofabout 1,280 gallons per minute from vessel 28 into vessel 16 where it isthoroughly mixed with the first slurry.

About 45 gpm of water and volatile materials (HF, SiF₄, H₂ S, CO₂ etc.)is vaporized from the second slurry in vessel 28. Vessel 28 ismaintained at about 15 inches of mercury absolute. Approximately 400 gpmof slurry is withdrawn from the second reaction vessel and flows tovessel 48, the separator feed tank. Thus about 445 gpm of material(vaporized material and the slurry to the separator feed tank) isremoved from the system. The removed slurry is then passed to theseparation section where the solid and liquid portions of the slurry areseparated.

At these rates, the plant will produce about 350 tons per day of P₂ O₅of 35-44% P₂ O₅ phosphoric acid. The recovery data is summarized below.

    ______________________________________                                        TOTAL LOSS IN FILTER CAKE                                                                      % of P.sub.2 O.sub.5 fed in rock                             ______________________________________                                        Citrate insoluable (CI)                                                                          0.76                                                       Citrate soluble* (CS)                                                                            4.64                                                       Water soluable (WS)                                                                              2.34                                                       Total loss         7.74                                                       Total recovery     92.26                                                      ______________________________________                                         *Lattice substitution losses determined using ammonium nitrate instead of     ammonium citrate solution.                                               

A typical analysis of the phosphoric acid produced by this process isshown in Table 4. The total residence time, from entering vessel 16 toexiting vessel 48, is calculated at 7.9 hours. The volume of vessel 16is about 120,000 gallons, the volume of vessel 28 is about 40,000gallons to normal liquid level.

Phosphate rock is present in the first and in the second slurries in thefirst and second reaction vessels respectively. The amount present isquite small and will vary considerably. The value for the "CitrateInsoluble" loss of the filter cake is a rough measure of undissolved andunreacted phosphate rock.

EXAMPLES 2 TO 7

The following system as described hereinafter was set up in a pilotplant to duplicate actual plant operation in order to investigate theeffect of defoamers and crystal modifiers on the filterability, andhence the crystal size, of the calcium sulfate hemihydrate produced.

Into a first reaction vessel containing reaction slurry was addedphosphate rock, recycled phosphoric acid and recycle reaction slurryfrom the second reaction vessel. Defoamer, when used, was added in thefirst reaction vessel. The reaction slurry so formed in the firstreaction vessel was circulated to the second reaction vessel. Sulfuricacid and crystal modifiers were added to the second reaction slurry. Thesecond reaction vessel was maintained under slight vacuum so as toremove gaseous impurities and water from the slurry. The evaporation ofwater was utilized to cool the reaction slurry.

The conditions employed in determining the utility of the crystalmodifier and the defoamer in early runs and later runs are shown belowin Table 5. Results of the tests from the later runs are shown in Table6.

Phosphate rock is present in the first and in the second slurries in thefirst and second reaction vessels respectively. The amount present isquite small and will vary considerably. The value for the "CitrateInsoluble" loss of the filter cake is a rough measure of undissolved andunreacted phosphate rock.

EXAMPLE 8

A hemihydrate system substantially as shown in FIGS. I and III andoperated substantially as described in Example 1 was discovered to bewasteful of product acid on start-up, that is, the first one or two daysof production could not be economically filtered due to fine particlesin the feed to the filter and had to be "dumped wet" with a resultingnew high loss of P₂ O₅ values.

In a modified start-up procedure, which is the invention of Gragg et al,the system was first filled with water heated by steam to 210° F. Thispreheated the vessels while testing the equipment at temperature. Thesystem was drained and filled with 185° F., 31% P₂ O₅ acid. Phosphaterock was fed to the system, while acid was recycled from the filter feedtank until the solids were builtup to about 30%, the sulfates wereadjusted to the proper level (e.g., negative in the dissolver andpositive in the crystallizer), and slurry was then sent to the filter.The plant was started up at 150 tons/day feed rate. The filter was linedout within 2 hours of startup (i.e., no wet pans after 2 hrs.). Within48 hours the design rate of 350 TPD was achieved with no apparentproblems. Product acid strength was up to 40% P₂ O₅ within one shift.

After the completion of a 44 day plant test and several modifications tothe dissolver, the plant was again started up. Essentially the samestartup procedure was used except the system was not preheated with hotwater and 35% acid @ 185° F. was used in the charging of the system. Thestartup went very smoothly with no particular problems, although thefilling of the system was slowed by evaporator problems. The firsthemihydrate cake that was produced filtered well. No filter pans weredumped wet during startup.

Upon completion of the plant trial runs in this Example 8, the averageparameters in Table 7 were calculated from data gathered over the entirecourse of the runs.

FIG. I illustrates a reaction system, which can be used either for ahemihydrate or gypsum process, similar to that of FIG. III but showingthe incorporation of a splitter box (to provide a head or vacuum sealand also a break to prevent siphoning) and a rock box (to trap hardpebble-like masses which apparently accumulate upon and break off fromthe sulfuric acid sparger in the crystallizer). Also shown are thecondenser between the crystallizer and the vacuum pump (which condensessteam and fluorine compounds), a scrubber to protect the vacuum pumpfrom fluorine compounds and an entrainment separator or disengager (toprevent nongaseous matter, mostly droplets containing fluorides, fromentering the vacuum pump) and the filter (which is the preferred meansof separating the calcium sulphate solids from the product phosphoricacid). The splitter box is a compartmented vessel with an entry linewhich can be moved to the top of either compartment (for slurry from thecrystallizer) at the top of one compartment and two exit lines, one atthe bottom of each compartment. One exit line is for conducting slurryto the filter feed tank, the other leads to a drain and is used forcleaning the line and the splitter box.

The present process and system can be used in practice of the inventionof Ser. No. 676,559, especially in a process for the production of highpurity phosphoric acid from phosphate rock containing metal impuritiescomprising calcium, magnesium, aluminum, ferric and ferrous iron asdescribed therein and in the applications of Ore et al, filed Dec. 29,1977.

                                      TABLE 1                                     __________________________________________________________________________                               Sulfate          Total                                                        Present          SO.sub.4                                             Stoichiometric                                                                        in 100%                                                                              Sulfate                                                                             Total                                                                             used as                           Rock                                                                              CaO  SO.sub.4  Sulfate (SO.sub.4) to                                                                 H.sub.2 SO.sub.4                                                                     Equivalent                                                                          Sulfate                                                                           a fraction of                     Fed in Rock                                                                            in Rock                                                                            CaO Fed                                                                            CaO in Rock                                                                           Fed to Unit,                                                                         Used  Used                                                                              Stoichiometric                    TPD %    %    TPD  TPD     TPD    TPD   TPD Amount                            __________________________________________________________________________    1209.5                                                                            44.90                                                                              0.65 543.07                                                                             930.97  844.4  7.86  852.26                                                                            0.915                             1383.1                                                                            45.97                                                                              0.65 635.81                                                                             1089.4  1052.2 8.99  1061.2                                                                            0.974                             1381.6                                                                            46.76                                                                              0.65 646.04                                                                             1107.5  1024.7 8.98  1033.7                                                                            0.933                             1172.2                                                                            46.81                                                                              0.65 548.71                                                                             940.64  844.3  7.62  851.9                                                                             0.906                             1110.9                                                                            46.89                                                                              0.65 520.90                                                                             892.97  804.6  7.22  811.82                                                                            0.909                             __________________________________________________________________________

                  TABLE 2                                                         ______________________________________                                        Phosphate Rock Analysis                                                                    For Example 1 For Example 8                                      Compound     % By Weight   % By Weight                                        ______________________________________                                        P.sub.2 O.sub.5                                                                            31.2          32.15                                              CaO          45.6          46.81                                              Fe.sub.2 O.sub.3                                                                           1.4           0.79                                               Al.sub.2 O.sub.3                                                                           1.2           1.12                                               MgO          0.4           0.37                                               SiO.sub.2    8.7           4.6                                                F            3.7           3.59                                               SO.sub.3     0.9                                                              CO.sub.2     3.6                                                              Organic      1.8                                                              H.sub.2 O    1.1           2.08                                               Na.sub.2 0,K.sub.2 O                                                                       0.4                                                              SO.sub.4                   0.79                                               ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Typical Phosphate Rock Screen Analysis                                        Mesh             Cummulative Percent                                          ______________________________________                                        +14              0.4                                                          +24              2.6                                                          +28              9.3                                                          +35              26.6                                                         +48              64.1                                                         +65              86.4                                                          +100            97.7                                                          -100            2.3                                                          ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Phosphoric Acid Analysis                                                                 Example 1   Example 8                                              ______________________________________                                        P.sub.2 O.sub.5                                                                            37.95          41.77                                             SO.sub.4     1.72          2.41                                               CaO          1.04          0.23                                               F            1.27          1.22                                               MgO          0.46          0.51                                               Fe.sub.2 O.sub.3                                                                           0.97          0.97                                               Al.sub.2 O.sub.3                                                                           0.91          0.84                                               Solids                     0.78                                               Sp. Gr.                    1.517                                              ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        GENERAL REACTION CONDITIONS                                                   (Pilot Plant)                                                                                   Early Runs                                                                             Later Runs                                         ______________________________________                                        Slurry Density      1.72 g/cc  1.72                                           Sulfate Concentration*                                                        First Reaction Vessel                                                                             -2%        -2                                             Second Reaction Vessel                                                                            +2%        +2                                             Phosphate Rock Feed Rate                                                                          174 g/min. 174                                            Slurry Recycle From Second                                                    To First Reaction Vessel                                                                          2200 g/min.                                                                              2200                                           Recycle Phosphoric Acid Feed                                                  Rate To First Reaction Vessel                                                                     390 g/min. 390                                            Sulfuric acid (93%) Feed Rate                                                 To Second Reaction Vessel                                                                         150 g/min. 150                                            Defoamer Feed Rate To                                                         First Reaction Vessel                                                                             0.7-1.1 g/min.                                                                           0.25-0.76                                      Crystal Modifier (CM) Feed Rate                                               To Second Reaction Vessel                                                     (1% Soln of CM in Water)                                                                          0.1-0.4 g/min.                                                                           0.006-0.76                                     Temperature of Slurry                                                         In Both Reaction Vessels                                                                          195-205° F.                                        50% Mass Dominant                                                             Crystal Size        30-40      26-43                                          ______________________________________                                         *By titration method                                                     

                                      TABLE 6                                     __________________________________________________________________________                                             Calcium Sulfate                                                               Hemihydrate 50% Mass                                                          Dominant                             Defoamer   Amount                                                                             Crystal Modifier                                                                             Filter Rate**                                                                           Crystal Size                         Example                                                                            Type  Wt. %                                                                              Type    Amount ppm*                                                                          TON P.sub.2 O.sub.5 /ft.sup.2 -day                                                      (microns)                            __________________________________________________________________________    1    None  None None    None   0.35 to 0.42                                                                            42.8                                 2    None  None None    None   0.70      39.1                                 3    None  None Actrasol W-40                                                                          0.1   0.85      42.8                                 4    AZ10A 0.071                                                                              Actrasol W-40                                                                         10.8   1.24      --                                   5    AZ10A 0.12 Actrasol W-40                                                                         12.3   1.72      --                                   6    AZ10A 0.058                                                                              None    None   0.80      35                                   7    AZ10A 0.061                                                                              Actrasol W-40                                                                         12.2   0.70      35                                   8    AZ10A 0.037                                                                              Actrasol W-40                                                                         10.0   0.45      26                                   __________________________________________________________________________     Actrasol W40  primarily sodium dodecylsulfonate, Arthur C. Trask Corp.,       Summit Ill. 60501                                                             AZ10A  Tall oil fatty acids and esters*, AZ Products Co., P.O. Box 67,        Eaton Park, Florida 33840                                                     *AZ Defoamer 10a is a proprietary blend of tall oil fatty acid and a          variety of surfactants.                                                       *Based on weight of slurry                                                    **15 in Hg, 11/4"coke thickness, wash to dry solids 1.02, 33% solids in       slurry, 40 to 44% P.sub.2 O.sub.5                                        

                  TABLE 7                                                         ______________________________________                                                             Run Average                                              ______________________________________                                        Temperature in Dissolver - °C.                                                                94-98                                                  Amount SO.sub.4 Added                                                         Stoichiometric H.sub.2 SO.sub.4 *                                                                    0.96-0.97                                              % P.sub.2 O.sub.5 in Product from                                             Crystallizer           44                                                     Temperature in Crystallizer - °C.                                                              98-101                                                Reactor Volume First Stage                                                    Reactor Volume Second Stage                                                                          2.87**                                                 ______________________________________                                         *Amount H.sub.2 SO.sub.4 required to convert Ca in Feed to Dissolver to       Calcium Sulphate including SO.sub.4 in Rock                              

We claim:
 1. A system for the preparation of phosphoric acid fromphosphate rock and sulfuric acid, including in combination:(a) a firstreaction vessel containing a first slurry comprising calcium sulfatehemihydrate, monocalcium phosphate and phosphoric acid, (b) a secondreaction vessel containing a second slurry comprising calcium sulfatehemihydrate, monocalcium phosphate, sulfuric acid and phosphoric acid,(c) means including a draft tube means in each of said vessels formaintaining a continuous circulation of the slurry therein through saiddraft tube means at a rate of at least 50% of the volume of the slurryin said vessel per minute, said last mentioned means including asinglewall draft tube disposed centrally in each of said vessels and anagitator means positioned axially in each of said vessels within saiddraft tube, whereby on actuation of said agitator means the slurry ineach of said vessels will flow from the bottom portion of said drafttube up through the draft tube and on exiting the top of the draft tube,the slurry will flow downwardly in the space between said draft tube andthe inner wall of the vessel, (d) a first conduit means interconnectingsaid first and second reaction vessels for conducting said first slurryfrom said first reaction vessel to said second reaction vessel, (e) asecond conduit means interconnecting said vessels for conducting saidsecond slurry from said second reaction vessel to said first reactionvessel, (f) means for providing reduced pressure to said second reactionvessel to effect temperature control in said second reaction vessel byevaporation, means to circulate slurry between said second reactionvessel and said first reaction vessel at a sufficient rate so as to adddesired sulfate values and phosphoric acid values to and to effecttemperature control in said first reaction vessel, and means to form avacuum seal between said first and second reaction vessels, (g) an inletpipe means for introducing a mixture of phosphate rock and phosphoricacid to said first reaction vessel, said inlet pipe means havingdischarge means to the interior of the draft tube in said first vessel,(h) means for introducing sulfuric acid to said second reaction vessel,(i) means for withdrawing a slurry containing phosphoric acid andcalcium sulfate hemihydrate from said second reaction vessel, and, (j) avent means connected to said inlet pipe means to permit escape of gasesand reduce foaming generated by the dissolving reaction in said firstvessel.
 2. A system as defined in claim 1, including a third reslurryvessel for reslurrying phosphate rock and recycle phosphoric acid, and athird conduit means interconnecting said third vessel with said inletpipe means to said first reaction vessel.
 3. A system as defined inclaim 2, said first vessel being a dissolver vessel for essentiallydissolving phosphate rock in said first slurry, said second evacuatedreaction vessel being cooled by evaporation and functioning as acrystallizer vessel for crystallizing calcium sulfate hemihydrate insaid second slurry, and including a fourth filter feed vessel, and afourth conduit means interconnecting said last mentioned means forwithdrawing slurry from said second reaction vessel with said fourthvessel, for conducting said second slurry containing crystallizedcalcium sulfate hemihydrate and phosphoric acid to said fourth vessel,and an agitator means in said fourth filter feed vessel for maintainingthe slurry therein in suspension.
 4. A system as defined in claim 3,including a surge tank means in said fourth conduit means.
 5. A systemas defined in claim 3, including filter means, a fifth conduit means forconducting slurry containing crystalline calcium sulfate hemihydrate andphosphoric acid from said fourth filter feed vessel to said filtermeans, for filtering crystalline calcium sulfate hemihydrate from saidslurry, and a sixth conduit means connecting said filter means with saidthird reslurry vessel for conducting filtrate containing phosphoric acidto said third vessel.
 6. A system as defined in claim 3, including arock box in said fourth conduit, for trapping any relatively largerocks, stones and objects in said second slurry.
 7. A system as definedin claim 3, said first dissolver vessel being positioned at an elevationhigher than said fourth filter feed vessel and including an overflowpipe from said first vessel to said fourth vessel for conductingoverflow slurry from said first vessel by gravity to said fourth vessel.8. A system as defined in claim 1, said second reaction vessel beingpositioned at an elevation higher than said first reaction vessel, saidsecond conduit means being an overflow conduit means permitting returnof said second slurry in said second vessel by gravity through saidsecond conduit means to said first slurry in said first vessel.
 9. Asystem as defined in claim 1, including a sparger means in the bottomportion of said second vessel below the draft tube therein, said meansfor introducing sulfuric acid into said second vessel comprising aninlet means, said inlet means being connected to said sparger means. 10.A system as defined in claim 1, including a first recirculation conduitmeans for selectively recirculating said first slurry from said firstvessel externally thereof and back to said first vessel, a first pump insaid first recirculation conduit means, a second recirculation conduitmeans for selectively recirculating said second slurry from said secondvessel externally thereof and back to said second vessel, a second pumpin said second recirculation conduit means, and valve means fordiscontinuing slurry flow in said first conduit means from said firstvessel to said second vessel during recirculation of slurry through saidfirst recirculation conduit means or during recirculation of slurrythrough said second recirculation conduit means.